Vitamin D₃ acts as a crucial biomarker for various diseases. Current methods for vitamin D₃ detection are time-consuming, expensive, and require trained personnel. We report a simple and cheap photodetector (PD)-based vitamin D₃ detection technique for the first time using Ag nanoparticles-covered Er-doped TiO₂ nanowires. The operational stability of the device was tested under the dark as well as the UV light illumination. Vitamin D₃ solution produced absorption bands at 222 and 280 nm, respectively. The PD current density varied from 1.29  ×  10^−  4  A  /  cm² to 1.74  ×  ^{10  −  4} at −4  V for pure ethanol solution and 20  pg  /  ml D₃ solution, respectively (under 220 nm illumination). The average absolute current values were reduced from 3.25 nA (220 nm) and 2.9 nA (340 nm) for 5  pg  /  ml to 2.95 nA (220 nm) and 2.54 nA (340 nm) for 30  pg  /  ml D₃ solution, respectively. The current gradually increased up to 3.34 nA (220 nm) and 2.7 nA (340 nm) as the concentration was increased up to 80  pg  /  ml. The PD current/vitamin D₃ concentration decreases exponentially from 0.58 to 0.03  nA  /  g  /  ml) for 5 and 80  pg  /  ml, respectively, under 220 nm excitation, from which an unknown concentration of vitamin D₃ can be obtained.